In recent years, automotive components made from cast aluminum alloys, such as wheels, control arms, rear knuckles, brake calipers, cross members and differential carriers, are finding wider application in the transport and automotive industries. Traditional heavy steel components, like wheels, are being replaced with parts made from cast aluminum alloy, as manufacturers seek to reduce vehicle weight in order to minimize fuel consumption and reduce exhaust emissions. For each 1 kg (2.2 lb) of weight saved, it is estimated that a reduction of carbon dioxide (CO2) emissions by 20 kg (44 lb) is possible for a typical vehicle covering 170,000 km (105,000 mi) of distance travel.
A light weight aluminum wheel not only has a mass reduction effect that reduces emissions and fuel consumption, but it also improves the overall driving performance, passenger comfort, and vehicle road handling characteristics of a vehicle because lighter wheels result in less rotational inertia requirement for the vehicle to accelerate and decelerate.
Additionally, wheels that are made from aluminum offer superior aesthetic appearance over their steel counterparts. Therefore, considerable global research and development activity is currently focused on improving the properties of aluminum wheels and reducing their processing costs.
Most aluminum wheels are currently manufactured from a casting approach or from a forging approach. When compared with a forged wheel, a cast wheel has the advantages of design flexibility and lower cost.
Currently, the most commonly used alloy for cast aluminum components, such as wheels, is the aluminum A356 alloy. The A356 alloy is 92.05% aluminum, 0.20% copper, 0.35% magnesium, 0.10% manganese, 7.00% silicon, 0.20% iron and 0.10% zinc by weight. The alloying elements magnesium and silicon are considered the major aging hardening solutes and contribute to the A356 alloy's increase in impact toughness and other mechanical properties when heat treated.
Since impact toughness is a combination factor of a material's strength and ductility, and it reflects the amount of energy absorbed by a material during impact or fracture, the impact toughness coefficient can be determined by measuring the area underneath the stress-strain curve. By taking the integral of the stress-strain curve, the impact toughness coefficient (M) represents the absorbed energy per unit volume, and the mathematical description is shown below, where: ε is the strain, εf is the final strain of the material upon failure, and a is the maximum stress value.
  M  =            Energy      Volume        =                  ∫        0                  ɛ          ⁢                                          ⁢          f                    ⁢              σ        ⁢                  ⅆ          ɛ                    
Other mechanical properties, such as a material's ductility, shear strength, shear strain and tensile strength may also be improved by the types and amounts of alloying elements added to aluminum.
It has been known that the high crash-worthiness performance for a vehicle wheel requires the wheels material to have high fracture toughness or high strain energy under impact. Unfortunately, the mechanical properties of A356 alloy, such as tensile strengths and ductility, are restricted by the coarseness of the microstructure and casting defects. In order to overcome the mechanical property deficiency, the prior art approach is to design cast aluminum wheels with much thicker cast sections than necessary in order to meet the wheels' crash safety requirements.
A thicker wheel cross sectional design in effect will result in a cast wheel of relatively heavier weight than wheels made from a forging approach. Therefore, there is a need to improve the prior art approach for aluminum A356 alloy.
It has also been a common practice by the casting industry to use the conventional T6 heat treatment in order to produce maximum strength in cast aluminum. The T6 heat treatment has two phases, a solution heat treatment phase and an aging phase. In the solution phase, the A356 (or other alloy being used) is heated to 1000° F. for at least 9 hours, causing the magnesium and silicon in the alloy to dissolve into the aluminum. This creates a single phase alloy containing the hardening agent of magnesium-silicide (Mg2Si). Prior to the aging phase, the A356 is then rapidly cooled by water quenching to prevent the Mg2Si crystals from re-separating within the alloy.
During the second phase, or aging phase, the A356 alloy is heated to approximately 310° F. for 10 hours and then air cooled, allowing the magnesium and silicon to form a uniform distribution of small Mg2Si precipitate crystals in nano-scale. This process is called precipitation hardening. The formation of the Mg2Si precipitates crystals increases the strength of the A356 alloy by up to approximately 30% by using the aging heat treating step.
The T6 heat treatment process is particularly suited for use with the low pressure permanent mold (LPPM) casting process known in the art. The LPPM process uses a permanent mold, usually made of iron or steel. Instead of using gravity to feed molten aluminum alloy into the mold, the LPPM process applies a low atmospheric pressure to the molten alloy, causing the metal to slowly flow into and fill the mold cavity without creating a turbulence air-liquid mixture flow. The LPPM process involves a directional solidification of the molten metal, which in turn results in a finer grain size and better alloy microstructures. For these reasons, the LPPM process creates higher quality castings with low tooling costs and allows for thin walled castings and castings with intricate designs, which are difficult to achieve using other casting processes. The LPPM process, itself, may also increase the mechanical properties of the material up to 10%.
One problem known in the art with the T6 heat treatment is the long time required to complete the process. The entire process of solutioning, quenching and aging could take up to twenty hours or more; therefore, it has a significant cost implication for mass production of cast aluminum wheels and other components.
To speed up the heat treatment process, semi-automatic drop bottom batch furnaces are preferred for premium grade aluminum castings. These drop bottom furnaces prove the shortest time for the water quench step. However, the solution, quench and age steps require specific controls for time and temperature. The controls are developed around a specific load size for the casting in the system at a given point in time.
It is important with the large batch heat treating quantities associated in high volume production to minimize the time difference from first part arrival at target temperature and last part arrival set at target temperature. A lesser time differential between these two events results in greater consistency in improving the mechanical and impact toughness properties of the castings.
Furthermore, after the required T6 heat treatment process to produce maximum strength, aluminum wheels are often powder coated or painted and allowed to dry for a period of time in an oven at a temperature range from 310° F. to 400° F. In order to save energy and time, occasionally the manufacturers would use this paint-bake temperature range, which is similar to that used for the T6's artificial aging process, in order to gain some of the heat treatment strengthening required for optimum mechanical properties. However, the short period of time required to bake the paint (typically less than one hour) is not sufficient to achieve alloy strengths close to the true T6 values without the proper aging process.
Therefore, there is a need for a new method to improve the prior art approach for heat treatment in conjunction with the thermal coating process, in order to yield high mechanical property with substantial energy and cost savings. In particular, there is a need to improve the impact fracture toughness coefficient of a cast aluminum component.
Various attempts to increase the toughness, strength and other properties of aluminum components cast using the A356 alloy have been made. In some instances, the composition of the A356 alloy is altered or variations are made to the T6 heat treatment. In other instances, a combination of both A356 alloy and T6 treatment modifications are used.
For example, U.S. patent application Ser. No. 12/683,186 (Wang, et al) discloses a method for strengthening cast aluminum components by modifying the aluminum alloy used to include 0.3% or more magnesium, 0.8% or more copper, 5% or more silicon and 0.5% or more zinc. However, to maximize the additional strength and toughness which may be potentially created with the modified aluminum alloy, the alloy must be treated using a two-stage solution treatment and a two-stage aging process. The 4-step process, which includes an initial heating, incremental heating, low temperature aging and high temperature aging, does not decrease the time and cost for processing cast aluminum components and results in only a 10% increase in the tensile strength.
The 4-step process is also a non-isothermal process and not applicable to the LPPM casting process.
Similarly, U.S. patent application Ser. No. 12/145,614 (Wang) discloses a modification of the T6 heat treatment method which increases the tensile strength of cast aluminum components by 10-15% while decreasing the heat treatment time by approximately 35%. However, this treatment method uses a non-isothermal process and is only applicable to the solution treatment, and not to the aging treatment. This method is also most suitable for the A319 alloy and will not achieve the same increased impact toughness coefficient for the A356 alloy, which is usually the material of choice for making aluminum wheels.
It is desirable to modify the A356 aluminum alloy to improve its impact toughness coefficient, when heat treated.
It is desirable to decrease the amount of processing time and thermal energy expended in heat treating an aluminum alloy.
It is desirable to decrease the cost of heat treating aluminum alloys.